Next generation military aircraft will require thermal management of much higher loads with minimal aircraft performance penalties. Such aircraft may include a thermal management system that incorporates a vapor cycle system (VCS). However, a VCS typically requires a low temperature heat sink for waste heat rejection.
Heat sinks that are typically available for rejecting waste heat include ram air, engine fan air and third stream air. A VCS can reject waste heat directly to ram air, but the accompanying increase in ram drag significantly degrades aircraft performance. Directly rejecting VCS waste heat to the engine fan air or third stream air is generally not a viable option because the temperature of these air streams is too high.
An air cycle machine can be used to generate a low temperature heat sink for a VCS. The air cycle machine would receive VCS waste heat via a refrigerant condenser and then transfer the water heat to either the engine fan air or third stream air. However, using air cycle machine for generating a VCS sink is extremely inefficient, resulting in a significant degradation in aircraft performance.